In recent years, not only information apparatus but also various apparatus (e.g., household electrical appliances such as a television and image recording device, and sound reproduction devices) have a built-in magnetic recording device. Accordingly, there are growing demands for magnetic recording devices.
In addition, the volume of information to be stored is also increasing year by year, which has fueled efforts to develop technologies for improving storage density of magnetic recording devices. Especially, in recent years, as storage density of magnetic recording devices increases, there is a need for improvement in performance of a magnetic head and a recording medium (hard disk etc.).
In order to increase storage density in a magnetic recording device, it is effective to configure a recording medium with smaller grains. However, magnetization of smaller grains is poor in heat stability. This may cause recording bit inversion even due to heat energy at room temperature.
In order to solve this problem, it is effective to increase anisotropy energy of the grains. However, high anisotropy energy of the grain increases a coercive force of a recording medium, so that it becomes impossible for an existing magnetic head to invert magnetization of the grain. This causes a problem that it is difficult to record information.
Solutions to these problems are disclosed in, for example, Patent Literatures 1 and 2. Patent Literatures 1 and 2 disclose a method called “optical assist magnetic recording”. In this method, a recording medium with high coercive force is used. In order to record information, an information recording region of the recording medium is irradiated with light so that a temperature of the light-irradiated region rises. This lowers the coercive force temporarily so that information is recorded. The recording medium which has recorded information is then cooled down so as to increase coercive force and thus increase heat stability.
In such optical assist magnetic recording, storage density of a recording medium depends on a size of the light-irradiated region. Therefore, near-field light is commonly used to increase the storage density. For example, a piece of metal including a near-field light emitting section is irradiated with laser light so as to excite a surface plasmon. This enables to emit near-field light in a region smaller than a diffraction limit of light, the region in the near-field light emitting section. Irradiating a recording medium with this near-field light enables to heat only a minute region of the recording medium.
In an optical assist magnetic recording device, a semiconductor laser is commonly used as a small device which can be manufactured at low cost. The semiconductor laser is one of the fundamental components of the optical assist magnetic recording device, and a characteristic of the semiconductor laser has significant influence on a characteristic of the optical assist magnetic recording device.
In general, a characteristic of the semiconductor laser is easily affected by a usage environment. For example, an increase in a drive temperature (ambient temperature) causes a higher oscillation threshold, lower optical output under a certain electric current, lower efficiency in light emitting, and other such problems. Also, even under an identical usage environment, a higher oscillation threshold and lower optical output are observed due to deterioration over time. Furthermore, the semiconductor laser is vulnerable to noise of a power supply, and input of an electric current, as noise, which is greater than a rated current may cause an irreversible deterioration.
FIG. 13 is a graph showing a relationship between drive time and drive current of a common semiconductor laser. FIG. 13 illustrates the relationship between drive time and drive current (electric current) in a case where the semiconductor laser is continuously used with a constant optical output of the semiconductor laser under constant ambient temperature.
As shown in FIG. 13, even under such a condition in which there is no power supply noise and no fluctuation of ambient temperature, drive current of a semiconductor laser tend to increase with drive time due to spread of a defect inside the semiconductor or the like. In addition, although the drive current increases linearly along with time before the drive time reaches a given drive time tz, the drive current tend to increase rapidly once the drive time exceeds the given drive time tz.
Therefore, a stable usage of a semiconductor laser is possible until the drive time tz. A life (durable period) of a semiconductor laser is generally set to be shorter than the drive period tz or a drive period tz estimated by a manufacturer. The life of a semiconductor serves as an indication for an end of usage of a semiconductor laser or a time to change elements.
Although a life of a semiconductor laser is commonly approximately 7,000 to 50,000 hours, the life of a semiconductor laser varies depending on an operating condition (especially, ambient temperature, a condition of a power supply, or the like). The life of a semiconductor laser tends to be shorter in a severer condition (e.g., higher ambient temperature, more power supply noise, or the like).
Assuming that a magnetic recording device is a built-in hard disk included in a computer of a PC or the like, an operating condition of magnetic recording devices varies with individual device. For example, a drive temperature of a semiconductor laser varies in accordance with a temperature of an environment where the computer has been installed, a cooling effect inside the computer, or other such conditions. Therefore, the life of a semiconductor laser is more likely to vary with individual semiconductor laser.
A malfunction of a semiconductor laser causes an optical assist magnetic recording device to record insufficiently. Furthermore, in a case where the above life of the semiconductor laser is shorter than a life of other machine parts included in the magnetic recording device, the life of the semiconductor laser can be a life of the optical assist magnetic recording device.
Accordingly, it is important to estimate a life of a semiconductor laser and to perform stable drive from the viewpoint of stabilization of recording properties of an optical assist magnetic recording device and data protection.
Conventionally, SMART (Self-Monitoring, Analysis and Reporting Technology) has been used for estimating a life (malfunction) of a hard disk. The SMART, for example, stores drive records of a hard disk under dozens of categories (e.g., the number of seeking and the number of reading errors). The SMART then performs a self-analysis of a characteristic of the hard disk based on the drive record. The SMART enables estimation of a life affected by deterioration over time under a stable usage environment. However, no category regarding a life of a semiconductor laser is provided to the SMART.
On the other hand, Patent Literature 2 discloses an arrangement for detecting a drive temperature of a laser element as a means of stabilizing light intensity of the laser element in an optical assist magnetic recording device. In the optical assist magnetic recording device disclosed in Patent Literature 2, optical output of the laser element is stabilized by feeding back the detected drive temperature of a laser element and optical output.
Moreover, Patent Literatures 3 and 4 disclose arrangements for estimating a life of a laser element from a value obtained by measuring a drive current and a drive temperature. Patent Literatures 3 and 4 also disclose devices which include the arrangements.
Specifically, a light-emission driving device disclosed in Patent Literature 3 calculates out, on the bases of a drive temperature obtained when a laser element is driven, deterioration threshold current corresponding to the drive temperature by using a temperature correction factor. If a drive current of a laser light exceeds the deterioration threshold current thus calculated out, injection of current is suspended so as to stabilize the light-emission driving device.
Furthermore, an optical disk device disclosed in Patent Literature 4 stores a temperature correction factor and a drive temperature, and calculates out accumulated time from a drive temperature of a laser element. If accumulated time of the laser element exceeds a threshold, the optical disk device bans high output drive (recording) of the laser element and permits only low output drive (reproduction). This enables the stabilization of the optical disk device.